Soil under pres­sure

Soil com­paction is a well-known prob­lem, but its impact is often under­es­ti­mat­ed and dif­fi­cult to assess. And it is a grow­ing con­cern for farm­ing stake­hold­ers.

At the end of Novem­ber it is bit­ter­ly cold in this cor­ner of the French départe­ment Aisne. Frédéric Sabre­ja, sug­ar beet grow­er, inspects the field. The weath­er is dry – hope­ful­ly it will remain so. “Last year we fin­ished har­vest­ing the beets in cat­a­stroph­ic wet con­di­tions,” he recalls. “The trail­ers were buried to their axles in mud. They left such ruts that while plough­ing a few days lat­er, the trac­tor almost bot­tomed out.”

Sim­i­lar con­di­tions in recent years are a con­cern to mem­bers of the machin­ery co-oper­a­tive Cuma (Co-opéra­tive d’utilisation de matériel agri­cole) of Cha­landry, of which Mr Sabre­ja is pres­i­dent. It affects both the soil struc­ture and crop yields, notes his col­league Nico­las Mal­lèvre, who accom­pa­nies him that day on a tour of their fields. “Even if it’s not extreme com­paction, we see the yield dif­fer­ence: Wheat after rape­seed yields 8-9t/ha, com­pared to 5t/ha after sug­ar beet. The lat­er sow­ing after beets does not account for all of that dif­fer­ence.”

When to har­vest?

Com­paction issues are increas­ing­ly wor­ry­ing pro­duc­ers in the North of France, espe­cial­ly with more fre­quent weath­er haz­ards, on nat­u­ral­ly sen­si­tive soils and with crops where excep­tion­al­ly heavy machin­ery is involved, such as sug­ar beet and pota­toes. How­ev­er, the prob­lem does not stop at the bor­ders. The Nether­lands, Ger­many, Aus­tria: On a glob­al scale, Europe as a con­ti­nent has the biggest soil struc­ture prob­lems in the world.

“When 70t machines are work­ing on sandy silt soils in heavy, wet con­di­tions, that scares me,” says Mr Mal­lèvre. “A sug­ar beet har­vester can com­pact the soil down to a depth of 1m. Earth­worms had bet­ter have strong shoul­ders to unpack that!” These con­cerns are shared by oth­er mem­bers of the Cuma co-oper­a­tive, par­tic­u­lar­ly giv­en the end of sug­ar beet quo­tas. The 20% increase in local sug­ar refin­ery pro­duc­tion and the result­ing extend­ed har­vest sched­ules make it increas­ing­ly dif­fi­cult to con­trol har­vest­ing con­di­tions. So in 2017 the Cuma mem­bers decid­ed to invest.

How far does it go down?

“We switched from con­tract­ing self-pro­pelled beet har­vesters on our 130ha of sug­ar beet to using a trac­tor with a front attach­ment to cut the foliage and a drawn beet har­vester,” reveals Mr Sabre­ja. “This allows us both to reduce the weight on the soil and, since we own the equip­ment, to har­vest in the best pos­si­ble con­di­tions.” The trac­tor-har­vester com­bi­na­tion has a net weight of 17t and Ultra­flex tyres are inflat­ed to 1.5 bar to spread the load. Hav­ing already adopt­ed pre­ci­sion tech­nolo­gies like auto guid­ance and sec­tion-con­trolled appli­ca­tion, mem­bers are now think­ing about switch­ing to remote tyre infla­tion sys­tems.

“The num­ber of axles is direct­ly relat­ed to the depth of com­paction, so we try to have as many as pos­si­ble on the machine’s chas­sis,” notes Mr Sabre­ja. In addi­tion, two rows of beets are sac­ri­ficed dur­ing sow­ing to allow for wide wheels to pass through.

“We do com­pact the soil, but try to stay as shal­low as pos­si­ble,” says Mr Mal­lèvre, point­ing at the beau­ti­ful beet field that stretch­es behind him. In oth­er words: Com­paction is linked to so many vari­ables that it can’t always be avoid­ed. It is there­fore more a ques­tion of con­trol­ling the risk. “If you com­pact deep­er than 30cm, it’s too late. Mechan­i­cal action is inef­fec­tive: Only deep frost, drought, or time will enable the struc­ture to improve again.”

We need to opti­mise as much as we can, to make farm oper­a­tions sus­tain­able.

Frédéric Sabre­ja

Cuma’s invest­ment in the new har­vest­ing sys­tem was log­i­cal – but the co-oper­a­tive still needs to stay eco­nom­i­cal­ly viable. The increase in beet pro­duc­tion has been accom­pa­nied by falling sug­ar prices, while oper­at­ing costs keep increas­ing. “Five years ago, we made €500/t (£438/t)”, says Mr Sabre­ja. “Today, we make €350/t (£306/t). There is no room for error: We need to opti­mise as much as we can, to make the farm oper­a­tions sus­tain­able.”

Fel­low farm­ers are gen­er­al­ly well aware of the risks of com­paction, he adds. But, for the time being, many do not see avoid­ing com­paction as a way to increase yields. How­ev­er, Mr Sabre­ja expects this issue to become more promi­nent in the years to come. “I think the impact is not yet notice­able enough.”

Soil com­paction in a nut­shell

What hap­pens in com­pact­ed soil? The dimin­ish­ing size of the pores and cav­i­ties begins to adverse­ly affect the exchange of air and water as well as plant root­ing. In the absence of a ver­ti­cal pore sys­tem, soil res­pi­ra­tion (O2 intake and CO2 removal) is no longer guar­an­teed, and water tends to stay on the sur­face. When there is an air capac­i­ty of less than 10% or a hydraulic con­duc­tiv­i­ty of less than 10cm a day, the oxy­gen defi­cien­cy will impact the micro-organ­isms and thus the for­ma­tion of humus. At the same time, plant roots find it more dif­fi­cult to access nutri­ents, which ulti­mate­ly ham­pers yield.

Source: Prof Dr Rain­er Horn. Read the whole inter­view here.

How severe is it?

350 km to the North, Louis Claessens, con­trac­tor in the Dutch vil­lage of Heino, describes the sit­u­a­tion in almost iden­ti­cal terms. “The prob­lem: Com­paction is not some­thing you see. As con­trac­tors, we leave the land nice and flat, which means it is impos­si­ble to see tracks that might have devel­oped below the sur­face. But the soil has actu­al­ly become so com­pact­ed that water can­not escape.”

Pas­sion­ate about agron­o­my, Mr Claessens pur­chased the con­tract­ing busi­ness ‘Volk­erink Heino’ in 2009. It was a dif­fi­cult time to start: The after­math of the 2008 finan­cial cri­sis was still being felt, and the risk was sig­nif­i­cant. Hence the idea of dif­fer­en­ti­at­ing him­self with ser­vices focused on soil analy­sis and preser­va­tion. “My strat­e­gy is to ful­ly utilise the poten­tial of the clients’ pro­duc­tion sys­tem. When they make mon­ey, so do we.”

In order to show his clients how seri­ous the risk of soil com­paction is, he recent­ly organ­ised a field demon­stra­tion, run­ning 14 tests with dif­fer­ent types of machines, chas­sis and tyres. “I want­ed to visu­al­ly demon­strate the impact that the wheel pres­sure has on the soil. Here, dairy farms do not care very much about soil qual­i­ty.”

The weight of the machine does not nec­es­sar­i­ly have a neg­a­tive impact on the soil.

Louis Claessens

At Volk­erink Heino, reduc­ing poten­tial soil com­paction dri­ves each invest­ment deci­sion. The Van der Molen eight-wheeled chas­sis was installed on the 16m3 slur­ry tanker as well as on one of the trail­ers. Tyre pres­sures are set at one bar and the load at the wheel remains below the 3t lev­el. Mr Claessens also uses a plough that allows the trac­tor to remain on the unploughed sur­face. “In terms of com­paction, con­trac­tors bear the bulk of the respon­si­bil­i­ty,” he says. “It’s up to us to do some­thing.”

One thing is cer­tain: Pre­vent­ing com­paction will be more of an issue in the years to come, against a back­drop of increas­ing machin­ery weights. “How­ev­er, the weight of the machine does not nec­es­sar­i­ly have to have a neg­a­tive impact,” says Mr Claessens. “Every­thing comes down to weight per wheel: It is bet­ter to car­ry 16m3 on eight wheels than 10m3 on two.”

Tech­ni­cal inno­va­tions are under way which, cou­pled with exist­ing pre­ci­sion tech­nolo­gies, should help farm­ers and con­trac­tors to apply meth­ods which allow the soil to breathe, and the plants to take root prop­er­ly.

Iden­ti­fy­ing com­paction

So how do you iden­ti­fy com­paction? Accord­ing to soil expert Philip Wright, farm­ers should first look for areas where the grass or crop is per­form­ing bad­ly – is it lying wet or yel­low­ing under stress? “Hav­ing iden­ti­fied prob­lem areas, dig a hole in a good area of the field to get a feel for what the soil struc­ture should be like, and then dig in the bad areas – you’ll be able to feel and see the dif­fer­ence straight away.” Dig a series of holes to at least 1.5-2 spade depths, then prise off some soil on a non-smeared face. It should break away ver­ti­cal­ly; if it frac­tures into hor­i­zon­tal slabs that’s an indi­ca­tion of com­pact­ed lay­ers. Look at the mois­ture in the soil pro­file, he adds. If there is a wet lay­er over a dry lay­er, water isn’t get­ting through. Also check the soil colour and smell, pres­ence of worms, and root growth. Are the roots grow­ing right down or reach­ing a lay­er and then stop­ping? All of this will help to iden­ti­fy the lev­el and depth of com­paction, so you can decide how to deal with it.

Alle­vi­at­ing com­paction

Crop roots can sup­port and improve soil struc­ture – and dry or frosty weath­er can offer nat­ur­al improve­ment through shrink­ing and crack­ing, says Mr Wright. “There is a temp­ta­tion to go in with a deep sub­soil­er or plough, but that will cost more in fuel, dis­rupt the nat­ur­al soil struc­ture and poten­tial­ly do more dam­age. You need to iden­ti­fy the lim­it­ing fac­tor in the soil and do just enough to alle­vi­ate it – use the met­al just enough to allow the roots to do the rest of the job.”